There has been conventionally a friction stir spot joining device for softening and stirring works to be joined due to friction heat generated by rotation of a pin so as to joint the works to be jointed together and provided with a pressing motor and a stirring motor fixed to one end side of an arm, a jointing tool having the pin at a tip end thereof and connected to an output shaft side of the stirring motor at a rear end thereof, wherein the stirring motor and the jointing tool are disposed on the same axis line (for example, as disclosed in JP-A 2002-126884).
However, in the prior art device, an axis of a ball screw for generating pressing force, the center of a directly moving guide (center of an elevating mechanism in the embodiment), and the center of a shaft of the stirring motor are largely eccentric from each other, an intermittent torque caused by a stirring force which accords with a rotating frequency of the stirring motor is applied to a guide portion of a pressing device to generate vibration, thereby shortening lifecycle of a guide of a directly moving ball of the pressing device, so that the size of the directly moving guide portion has to be made large, leading to an increase of cost. Further, there is a high possibility that the guide portion of the pressing device interferes with a narrow portion surrounded by the works to be joined and a jig, causing the joining device difficult to move to the narrow spot. Still further, since the pressure application center of a pressing force and a rotating center of a stirring shaft of the stirring motor are eccentric from each other, a tipping torque is generated at the pressure application center of the pressing force when the stirring motor is moved to press toward the works to be joined, thereby applying an excessive load to the guide.